A hologram is a record of an interference fringe of coherent laser light on a photosensitive material, and is used in various field, such as an optical device, a three-dimensional display, interferometry and processing of an image and information, owing to the multi-functionality thereof.
As a representative example of the conventional hologram recording material composition, a gelatin dichromate photosensitive material and a breached silver salt photosensitive material (described, for example, in Display Holography Handbook, p. 66 to 67 (Gyoin Shokan, 1985) and Optical Engineering Handbook, p. 351 to 353 (Asakura Shoten, 1986)).
However, although gelatin dichromate has a high diffraction efficiency, and a silver salt photosensitive material has a high sensitivity, these materials require a complicated process on production of a hologram, and particularly they have a problem in that they require a wet development process.
As a photosensitive material to eliminate the problem, a hologram recording material composition containing a photo-polymerizable monomer is proposed. In this material, a photo-polymerizable monomer is polymerized in a portion having a large light amount in the interference fringe to cause a refractive index modulation in that portion, and thus a hologram is recorded. Examples thereof include a photo-polymerization type recording material mainly comprising cyclohexyl methacrylate and N-vinylcarbazole as photopolymerizable monomers; and a photo-polymerization initiator, and a photo-polymerization type recording material mainly comprising butyl methacrylate and ethylene glycol dimethacrylate as photopolymerizable monomers, and 1-phenylnaphthalene as an inactive component not participating in the polymerization; and a photo-polymerization initiator (as described in Appl. Opt., vol. 15, p. 534 (1976), U.S. Pat. No. 3,993,485 and the like).
However, because these materials are in a liquid state, the composition flows between two surface materials on recording a hologram to prevent recording in good conditions. Furthermore, an unreacted monomer remains after recording the hologram in a portion of a small light amount, and therefore the record is necessarily stabilized by conducting an exposure treatment on the whole surface.
In order to solve the above-mentioned problems of the prior arts, it is proposed that a polymer which does not participate in photoreaction is used as a binder in order to inhibit fluidization of compositions, and the polymer is used in combination with a photopolymerizable monomer which is liquid at an ordinary temperature (U.S. Pat. No. 3,658,526). These compositions do not require treatment such as heating before and after recording holograms and can prepare the holograms only by simple dry treatment. However, this technique is inferior to the above-mentioned gelatin dichromate and silver salt photosensitive material in diffraction efficiency, which represents essential performance of the holograms.
As improvement of the above-mentioned prior art, compositions consisting essentially of a binder polymer, a liquid photopolymerizable monomer having a high refractive index, a plasticizer and a photopolymerization initiator, the photopolymerizable monomer containing an atom which contributes to the high refractive index such as chlorine or bromine in order to improve refractive modulation are proposed (U.S. Pat. Nos. 4,942,102 and 4,942,112 and the like). In the case of these compositions, compatibility with the plasticizer was often poor, and there were problems such as insufficient solubility of the compositions, haze and the like. Since recorded holograms had insufficient diffraction efficiency, it was necessary to perform heat treatment or the like after recording again to amplify the diffraction efficiency, and post-treatment was still troublesome.
As improvement of the above-mentioned prior art, compositions comprising a polymeric binder, a radical polymerizable compound having a 9,9-diarylfluorene skeleton and being liquid at an ordinary temperature, a plasticizer and a photopolymerization initiator are proposed (Japanese Laid-open Patent Publication No. 301322/1994). The publication discloses that since the radical polymerizable compound does not contain chlorine, bromine or the like and has the 9,9-diarylfluorene skeleton in order to increase a refractive index, the compound is compatible with components such as the binder polymer.
However, there existed the problem that since the compound is liquid at an ordinary temperature and has a lower refractive index than a monomer which is solid at an ordinary temperature, sufficient diffraction efficiency is not obtained in holograms after recording. Accordingly, a combination of the monomer having a higher refractive index and being solid at an ordinary temperature with the polymeric binder was desired. However, there existed the problem that when the monomer which is solid at an ordinary temperature is used, the photosensitive material causes haze, and sufficient optical transmission is not obtained.